SUMMARY / ABSTRACT This project addresses what is perhaps the biggest challenge in the clinical care of Type 1 diabetes (T1D): the incredible rise in HbA1c levels seen during the adolescent age span of 11-15 years. This rise appears to be driven by physiologic changes (an increase in insulin resistance) that are not adequately addressed, in part because of psychosocial challenges (including family conflict, peer influence, and diabetes distress). We propose to follow adolescents longitudinally for two years, with assessment of these physiologic factors and exploratory aim following psychosocial factors, as well as testing a promising intervention in the artificial pancreas (AP). This study is titled A.P. APPLE for Artificial Pancreas Adolescent Physiology and Psychosocial Longitudinal Evaluation. We hypothesize that long-term use of an AP system (Tandem's Control-IQ, compared to Usual Care+CGM) will in part blunt the rise in HbA1c that is typically seen. However, even apart from testing this intervention, this project offers valuable observational data on the natural history of changes seen in participants in the control group. We will follow changes in insulin resistance by body compartment (liver vs. muscle/adipose tissue) during pubertal progression using stable-isotope triple-tracer mixed meal tests, revealing novel information about how puberty-related insulin resistance evolves over time by body compartment?as well as how the AP might respond to these changes. We will also follow key psychosocial processes of family conflict, peer influence, diabetes distress, depression and quality of life to evaluate their temporal relationships to sub- optimal control. Following these processes may help in the application of pharmacologic or psychologic approaches to improve adolescent control?either with or without AP use. Finally, we will follow modern markers of T1D control, including time-in-range and glycemic variability to document for providers how these factors change during pubertal progression and how they are related to physiologic and psychosocial changes. This study takes advantage of the University of Virginia Center for Diabetes Technology as an environment with expertise in each of the domains of glycemia, physiology and psychosocial factors?as well as development of an AP system that is effective in short-term trials at reducing HbA1c. The sub-optimal control during adolescence has gotten more pronounced in recent studies and contributes to long-term complications. This project will provide valuable clues about the durability of AP use among adolescents as well the contribution of rising insulin resistance and psychosocial barriers?with a long-term goal of improving control and long-term health of adolescents with T1D.